Melt viscosity and crystallization rate as a function of temperature are often critical to the process of injection molding semicrystalline engineering thermoplastics.
The melt viscosity is critical in that it governs the mold fill of the part (lower melt viscosity leads to faster mold fill and the ability to fill smaller parts) and therefore it is desirable to minimize this important parameter.
The crystallization rate as a function of temperature is critical in that it controls the optimum mold temperature and cycle time of the process. It is desirable to operate at mold temperatures of less than 110.degree. C. since this allows for the use of traditional water heated, as opposed to oil heated molds. It is also economically desirable to operate at such a temperature as to allow for the optimum crystallization rate as this translates into shorter cycle times.
The use of a plasticizer is generally known for some polyesters to enhance both of these critical parameters. Polyalkylene ethers are known plasticizers for low melting polyesters such as poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET), but are known to be sensitive to thermal degradation. Polyalkylene ethers are reported to degrade when temperatures exceed 250.degree. C., as stated by W. K. Witsiepe (Adv. Chem. Ser., No. 129, 39-60, 1973). A plasticizer will typically decrease the melt viscosity and depress the glass transition temperature of the thermoplastic which in turn allows for a faster crystallization rate at a lower temperature.
Common plasticizers for polyester engineering plastics are low molecular weight organic esters such as neopentylglycoldibenzoate (Benzoflex S312) and dipropyleneglycoldibenzoate (Benzoflex 9-88).
The use of plasticizers in semicrystalline engineering polymers to improve moldability is well known to the art. Furthermore the need for and use of improved plasticizers for semicrystalline polyesters is well known to the art.
Polyalkyleneethers have been used as plasticizers for low melting polyesters such as PBT and PET, but are known to be sensitive to thermal degradation. In fact, U.S. Pat. No. 4,548,978 to Garrison, Jr., U.S. Pat. No. 5,028,647 to Haylock et al, U.S. Pat. No. 4,914,145 to Tohdon et al, U.S. Pat. No. 4,558,085 to Lee, and U.S. Pat. No. 5,004,817 to Bastioli et al and JP Patent 1,256,562 to Hara et al teach the use of polyalkylene oxides such as polyethylene glycol as plasticizers in polyethylene terephthalate.
However, E. A. Flexman reports in Adv. Chem. Ser., 233(Toughened Plastics I), 79-104, 1993 that many additives utilized to improve the properties of low melting polyesters, such as PBT, are not useful in higher melting polyesters because the significantly higher processing temperatures required degrade the additives.
Polyalkylene ethers degrade when temperatures exceed 250.degree. C., as reported by W. K. Witsiepe (Adv. Chem. Ser., No. 129, 39-60, 1973). In fact, the flash point of polyethylene glycol is reported to be 243.degree. C., attesting to its poor thermal stability. Polytetramethylene glycol, when exposed to high temperatures, undergoes a degradation reaction. Therefore, it would be expected that polyalkylene ethers would not be useful as plasticizers for semicrystalline polymers with melting points in excess of 250.degree. C.
It is reported by Witsiepe (Adv. Chem. Ser., No. 129, 39-60, 1973) that the optimum reaction temperature for polyether esters is 250.degree. C. because above this temperature degradation of the polyalkylene ether occurs.
In U.S. Pat. No. 4,438,233 discloses the practice of "end-capping" the polyalkylene ether to increase the thermal stability so that it can be processed at higher temperatures but not exceeding 200.degree. C.
U.S. Pat. No. 3,763,109 to Witsiepe and U.S. Pat. No. 3,856,749 to Hoeschele both describe methods of stabilizing polyalkylene ethers to high heat aging. These patents teach the necessity of added stabilizers to provide the polyalkylene ethers enough thermal stability to withstand temperatures of up to 170.degree. C.
Furthermore, U.S. Pat. No. 4,541,884 to Cogswell teaches the necessity for the plasticizer to be melt stable at least to the processing temperature of the matrix polymer.
U.S. Pat. No. 5,389,710 is of interest which discloses a polyester composition comprising a polyester such as PCT or PET and an effective amount of a certain type of .alpha., .omega.-bis(aminoalkyl)-polyoxyalkylene crystallization modifier which is required to chemically react with the PCT or PET composition. These modifiers require an amine group.